1. Field of the Invention
The present invention relates to a method of producing a liquid olefin polymer, and more particularly, to a method of producing a liquid polymer having a high viscosity index, a high flash point and a low pour point and having a very low viscosity, by polymerizing an .alpha.-olefin with the use of a specific catalyst.
2. Description of the prior art
Generally, lubricating oils are roughly classified into petroleum oils, synthetic lubricating oils, fatty oils, etc., but their fields of use are expanding, so that a synthetic lubricating oil is required which has high performance characteristics such as a high flash point, a high viscosity index and a low pour point, and so on. For instance, the lubricating oils used for jet aircraft are desired to have said high performance characteristics under both conditions of extremely low temperatures and higher temperatures.
In automatic speed change gears, high temperature hydraulic machines and other fields, lubricating oils having high performance characteristics are similarly needed. However, petroleum lubricating oils used most widely do not have the combination of properties desired for those specific applications. Attention is being focused on olefin polymer oils from the class of synthetic lubricating oils. The olefin polymer oils are produced by polymerizing olefins, and as the polymerization methods, cationic polymerization using Lewis acids such as aluminum chloride and the like, radical polymerization using heat or peroxides are generally enumerated, but the polymer oils having the desired high viscosity index have not yet been obtained because the polymerization is accompanied by the isomerization reaction of the olefins.
Hence, recently the use of the so-called coordinated anionic Ziegler catalyst that regularly effects the polymerization has become spotlighted, for instance, C.sub.8 - C.sub.10 .alpha.-olefins are polymerized with the use of a complex prepared from monoethyl aluminum dichloride and titanium tetrachloride as a catalyst to obtain a liquid polymer having high performance characteristics, namely, a viscosity index above 130. a flash point above 210.degree.C and a pour point below -50.degree.C.
However, the olefin polymer oils (lubricating oils) produced with the use of the usual Ziegler catalyst as it is, cannot satisfy the requirements, for instance, for working oils for aircraft as specified in MIL-H83282 or lubricating oils for jet aircrafts as specified in MIL-H7808 in (U.S. Army Standard). Consequently, in order to pass said standards, said olefin polymer oils are fractionally distilled by molecular distillation. But the distillate satisfying those requirements comprises only 30 - 40% of the starting polymer oils, and therefore, it cannot be said that the method is effective when 60 - 70% of the starting polymer oils is not used effectively.
On the other hand, the nature of the liquid olefin polymers of low viscosity grades is similar to that of squalane which is used in cosmetic compositions. It is desired to replace squalane with liquid olefin polymers because of the unstable supply of the raw material for preparing squalane.